As .beta.-mercaptopropionic acid is suitable for cross-linking agents for acrylic ester polymer, hardening agents for epoxy resin and the like, the production of .beta.-mercaptopropionic acid has recently increased rapidly. Relating to the production of .beta.-mercaptopropionic acid, there have been known several methods for making the acid by chloridizing a starting material, and by using thiourea or sodium hydrosulfide, e.g., in Japanese Patent Open Publication No. 198460/83. Correspondingly, I have already proposed a new making method by reacting acrylonitrile with sodium sulfide, in the presence of free sodium hydroxide, as disclosed in Japanese Patent Publication No. 6545/88 (i.e., Japanese Patent No. 1458518). This method is much economical because the process of production is simpler and also the time required for the total reaction is shorter than the known method mentioned above using acrylonitrile and sodium hydrosulfide.
According to my method described in Japanese Patent Publication No. 6545/88, ammonia gas, mirabilite solution and thiodipropionic acid are prepared evidently as by-products. When this ammonia gas is absorbed into water, it changes available pure ammonia solution, and when this mirabilite solution is crystallized on cooling, the crystal can be used directly to several different applications. When this thiodipropionic acid is also esterized with 2-ethyl hexyl alcohol, stearyl alcohol or the like, the ester has previously been useful as stabilizers for polypropylene or antioxidants for several synthetic resins. However, with respect to these applications, the demand of thiodipropionic acid has a decreasing tendency because another low-priced chemical compound is beginning to be used recently. It is probable that the disposal of thiodipropionic acid becomes difficult in the future.